Cathode heater insulation



Sept. 14, 1937. e. R. SHAW CATHODE HEATER INSULATION Filed Nov. 23, 1935 INVENTOR. GEORGE R. SHAW ATTORNEY.

Patented Sept. 14, 1937 UNITED STATES PATENT OFFICE CATHODE HEATER INSULATION Application November 23, 1935, Serial No. 51,211

6 Claims.

My invention relates to indirectly heated thermionic cathodes and to heaters for such cathodes, and more particularly to refractory insulation for cathode heaters and to an improved method of insulating them.

In indirectly heated cathodes of a common type the heater wire of tungsten or similar metal has been insulated with a coating of substantially pure sintered alumina or of alumina mixed with a small percentage of silica or tale to act as a binder. The electrical leakage or conduction of current through the insulation has heretofore been tested by measuring the current whichfiows in response to a unidirectional potential between the cathode and the heater at operating temperature. By this test, heaters With the sintered alumina insulation have the least electrical leakage, usually less than 3 microamperes at 50 volts difference of potential. Such heaters are satisfactory for radio tubes in the conventional receiving circuits, but recently radio receiving circuits have come into use in which there may be alternating potentials as high as 100 volts or more and of high frequency, usually 1000 kilocycles or more, between the heater and the cathode. It has been found that the usual sintered alumina insulation, in spite of its very high resistance and very low leakage with unidirectional potential, has low impedance and permits appreciable leakage of current in re-- sponse to alternating potential between the heater and the cathode. This leakage current is objectionable if excessive and may cause hum in the loud speaker of the receiving set.

One object of my invention is to provide an indirectly heated cathode in which the heater to cathode leakage is practically negligible at alternating potentials, even of high frequency, such as a kilocycle or more, between the heater and the cathode.

Another object of my invention is to provide a refractory composition with which a heater coil may conveniently be coated to provide an insulating layer having much higher impedance at all frequencies than the insulation heretofore available.

Still another object of my invention is to provide a method of insulating a cathode heater to make the impedance between the heater and the cathode very high and the leakage or electrical conductance negligible at alternating potentials such as are commonly used in radio receiving circuits and even at high frequencies.

Other objects and advantages of my invention will appear from the more detailed description of my invention.

For a better understanding of my invention reference may be had to the accompanying drawing, in which Figure 1 is a longitudinal section of an indirectly heated cathode, and Figure 2 is a perspective view of the cathode heater.

The cathode assembly illustrated in the drawing is of a conventional type in general use, and comprises a tubular cathode I of nickel or similar metal coated on the exterior with oxides of high electron emissivity, such as the conventional coating of barium and strontium oxides. The cathode is heated by a heater comprising a wire 2 of tungsten or similar metal coiled into a reverse coil and mounted inside the tubular cathode. The heater wire is insulated from the oathode by a layer or coating of refractory insulation interposed between the wire and the cathode, preferably an insulating layer or coating 3 on the coiled tungsten wire 2. In use the heater is connected to a source of heating current, usually cycle alternating current, which maintains the heater at a normal operating temperature considerably higher than the operating temperature of about 850 C. of the cathode I.

For practical purposes an indication of the value of the heater to cathode leakage on alternating potential, and hence of the impedance of the insulation may be obtained by bringing the heater to normal operating temperature with 60 cycle alternating current, connecting the oathode to either end of the heater wire through a resistor of about 250,000 ohms, and measuring the voltage drop across the resistor in millivolts with a root mean square vacuum voltmeter having a grid resistor of about 1 megohm and connected to the resistor through a condenser. With the conventional heater designed to operate at 6 volts there is during this test an average difference of potential of about 3 volts at 60 cycles between the heater and the cathode, causing an impedance leakage current to flow through the insulation and through the resistor and produce the voltage drop which is measured and which is dependent on the value of the leakage current. The voltage thus measured I shall, in accordance with the custom in the art, refer to as the heater-cathode hum voltage. The absolute valueof the heater-cathode hum voltage depends on the value of the resistor as well as on the value of the leakage current, but a resistor of 250,000 ohms gives values of heater-cathode hum voltage which are convenient and are in general use.

The impedance at high frequencies, such 1000 kilocycles, may also be measured with reasonable accuracy by a substitution method. One way of making this measurement is to use a radio frequency oscillator which oscillates at about 1000 kilocycles, and .is very loosely coupled to a tuned circuit in which a voltage of about 0.4 or 0.5 volts is developed. To calibrate the testing'set a resistor of known value, such as 1 megohm, and

preferably of the. metallized type, is connected across the tuned'circuit, the circuit tuned to' resonance, and thevoltage across the resistor read by means of the vacuum .tube volt, meter above mentioned. This procedure is repeated with other resistors of difierent values to obtain 1 readings which correspond to resistors of dif-* The heater cathode assem-' ferent' irnpedances. bly to be tested is connected acrossthe tuned circuit in place of the calibrating resistor, the

circuit tuned to resonance while the heater is kept at normal operating temperature, and the voltageof'th'e tuned circuit across the cathode assembly is read. This reading, when'compared with the readings obtained, from the various resistors used in calibrating the'test set, will show with reasonable accuracy, the impedance in megohms of thelcathode assembly under test,

With the same testseta measurement of the current which flows between the heater and the cathode at high frequencymay be obtained by 0 impressing between the. heater and the cathode a constant biasj'orvoltage of about '45 volts,

either positive or negative, and'then, also inpressing between the heater. and the cathode the highfrequency voltage obtained from the tuned 5 circuit of thetest set. The bias of the heater 'to the cathode is varied to.obtain minimum our-- 7' rent, which is taken .as the heater-cathode hum current at l000kilocycles. 1

j Indirectly heated cathodes having heaters insulated with sintered alumina insulation may show on the. average on these tests heater-cathode hum voltages of seven or eight hundred milli- V voltsor more, impedances of about 0.1 megohms,

land. heater-cathode hum currents. of above 500 milliamperes. If the heater insulation is alumina and hum currents may be considerably greater 7 and the impedance much less; but if the heaters are insulated in accordance with my invention,

0 the heater-cathodehumflvoltages will on the average be'hnly 101to millivolts, the impedances from 1 to 3 megohms, or higher, and the heatercathode hum currents from 20 to 100 milliam peres. r T 1 In accordance with my invention an insulating composition consisting of chemically pure alumina mixed with a small amount, such as A to 3%, of an additivematerial, preferably calcined and converted into the coherent layer or coating 3 by sintering the composition on the wire at high temperature. I have secured the best results in practice by adding to the alumina a compound of barium, particularly calcined barium carbonate which has. been fired in air. A convenient method is to-powd'er both the alumina and thea'dditive material and mix the powders thoroughly. This 'mixture may be further coated heater wire fired, preferablyin wet-hy drogemat about '1700'C. to 1800 ;C., which sintersthe coating. The extent to which themate-.

containing refractory binderzsuch as silica, or its compounds, the heater cathode' hum voltages,

barium carbonate, is applied to the heater wire ground into a very fine-powder, mixed with, an; organic binder, the heater wire. coated with-itby ;Well-known methods, such as spraying, and the' rials are ground in preparing the insulation is not particularly critical, but the grinding is preferably continued until the average particle size,

particularly in the final powder which is to be mixed with the binder, is about 1 to 5 microns.

In preparing the insulating material in accordance with my invention, I prefer to mix calcined and powdered'barium carbonate, preferably chemically pure, with powdered aluminum oxide, as free as reasonably possible from impurities, especially alkalis, and usually referred to as chemically pure, fire the mixture in hydrogen at 1600". C. to'1700 C., and grind the fired mixture to a very fine. powder.

One way of obtaining sufiiciently pure alumina from bauxite or similar ores is by the well-known Bayer process, in which in general'the bauxite is treated in concentrated sodium hydroxide solution 'in water at 170 C. under' pressure.

The alumina goes into solution, the iron and other impurities remaining as insoluble compounds. The solution'is filtered oif, and from it is ob-' tained a precipitate consisting principally of aluminum trihydrate which, when heated in air at about 1200 C., is converted into alumina. Alumina of substantially this degree of purity may also be purchased on the market as bauxite aluminum ore, concentrated, special purity, and

used. The calcined barium carbonate is of about the same density as aluminum oxide, and will mix satisfactorily with it. It would seen that the barium compound could be added in the form of barium carbonate, butI have found that the results are so much better when the barium carbonate is first calcinedand the barium is added to thealumina as calcined barium carbonate that for commercial use'the calcining of the bariumcarbonate may be essential. n

The aluminum oxide powder is thoroughly mixed with the calcined and powdered barium" carbonatelto form a mixture of 'which the powder obtained from the calcined barium carbonate constitutes from to 5%, and preferably about A ball mill is aconvenient de- 1% by weight. 7 Vice for mixing powders. For example, about 800 grams of thepowdered aluminum oxide and 8 grams of the calcined and powdered barium 'car- V bonate may be ball milled for about 2 hours with about 2500 grams of 'fiint'pebbles, to. mix the m powders, and the Imixture then fired atabout 1600 C. in hydrogen for about one hour, preferably in molybdenunilooats' The fired mixture is sieved through a 30 ;mesh sieve, andonly the material which passes through'the sieve rap- 1 idly is used. The fired and sieved mixture is then ground intovery-fine'powder in aball'mill with about three times its weight of mullite balls for about .18 hours and the'powdered insulation' is 7o ready'for use. a V n The finely powdered insulation maybe applied to the heater wire in practically the same way as the alumina insulation heretofore; used. --A con- -venient way is to mixthe powdered insulation With'a viscous organic binder. suchas is commonly used in this art. I prefer to make a suspension of the powdered insulation by mixing it with about twice its weight of a binder such as nitro-cellulose dissolved in a solvent like amylacetate. The insulation in suspension may be applied to the heater wire either by dipping or preferably by spraying to form a coating which should be uniform. The spray process for coating heater coils may be used on coils without a rigid core rod, if the coils are flexible enough so that the pressure from the spray gun moves them during the spraying to such an extent that the coating gets in between the turns. During the spraying all of the solid material in the coating preparation should be in suspension, the spray gun held close to the coils, and the spray adjusted to be sulficiently wet to obtain a smooth coating which adheres well to the wire and still be dry enough so that when adjacent turns stuck together by the coating are separated the coating does not break loose at the surface of either turn, but at the outer surface of the layers of coating. A wet spray is indicated by a glossy surface, while a dry spray is indicated by a dull or matte surface. The spray coated coil is dried in air, or preferably in an oven for three minutes at about 100 C., after which it is fired in a hydrogen or vacuum furnace, preferably in a molybdenum boat. The coated coils are fired for from four to eight minutes in the heating zone of the furnace maintained at a temperature of about 1700 C.

For a cathode heater of tungsten wire of a length and size suitable for operating at 6.3 volts and 0.3 amperes wound into a reverse coil about 25 mm. long and about 2 mm. in diameter, I prefer to use about 8 to 9 megohms of the coating preparation made and applied as above described.

Insulation consisting of alumina and about 1% of calcined barium carbonate forms on the tungsten wire of the finished heater a hard adherent coating, in which I find about 0.3% to 0.4% of barium by chemical analysis, indicating that roughly about 60% of the metal in the added material is lost during the manufacture of the insulation as above described. Somewhat the same percentage of loss occurs with other amounts of calcined barium carbonate so that the percentage of barium found by analysis may range from to slightly over 1%. If the manufacture of the insulation is carried on under conditions such that there is less loss of the added metal, the percentage of added metal compound may be correspondingly reduced, and if none were lost, the addition of about 0.4% of calcined barium carbonate would mean the presence of the desired 0.3% barium in the sintered insula tion of the finished heater. I have not determined in what form the barium found by chemical analysis is present in the insulation, but it is not present as metal, but as some refractory barium compound stable at the firing temperature, and is probably combined in some way with alumina, perhaps as a barium aluminate.

I have found that in practice the best and most consistent results are obtained with calcined barium carbonate added to the alumina. An addition of about 1% of calcined barium carbonate produces insulation with which a heater can be made having a heater-cathode hum voltage of from 10 to 50 millivolts when measured as above described, and the results are sufficiently consistent and controllable to permit use of the insulation in commercial production. For practical purposes the lower and upper limits for the calcined barium carbonate in insulation prepared as above described appear to be about 0.1/2% and 3%. Insulation outside these limits has much higher heater-cathode hum voltages on the test above described than insulation within these limits. Calcined carbonates of calcium, strontium, or magnesium may be used instead of calcined barium carbonate with fairly good results if suitable amounts are used. Lithium carbonate acts much the same as barium carbonate and may be used in much the same way and in much the same proportions.

What I claim as new is:

1. An indirectly heated cathode assembly comprising a tubular cathode sleeve coated on the outside with oxides of high electron sensitivity, a reversely coiled tungsten wire inside said cathode sleeve, and a coherent insulating coating on said wire consisting of chemically pure alumina and of a refractory oxygenic barium compound from A;% to 1% of the coating by weight of barium and having at normal heater operating temperature a heater-cathode hum voltage of less than 100 millivolts.

2. An indirectly heated cathode assembly comprising a cathode having a coating of electron emissive oxides, and a heater for said cathode comprising a wire of high melting point metal insulated from said cathode by coherent insulation containing an oxygenic barium compound and of which to 1% of the coating by weight is barium and the remainder chemically pure alumina, and having at normal heater operating temperature sufficiently high impedance between said heater and said cathode to produce a heater- 1 cathode voltage ranging between 10 and millivolts.

3. A heater for indirectly heated cathodes comprising a heater wire of high melting metal, and a coherent sintered insulation coating on said Wire of which substantially 99% is chemically pure alumina and the remainder an oxygenic barium compound stable at 1700 C. and containing from to 1% of the coating by weight of barium by chemical analysis.

4. An indirectly heated cathode heater comprising a tungsten wire, and a sintered coherent insulating coating on said wire consisting of chemically pure alumina and an oxygenic barium compound stable at 1700 C. and of which 0.3% to 0.4% by weight of the coating is barium.

5. An indirectly heated cathode heater comprising a tungsten wire, and a sintered coherent insulating coating on said wire consisting predominantly of chemically pure alumina and the remainder of an oxygenic compound of a metal selected from the group of alkaline earth metals and lithium, said metal being /4% to 1% of the coating by weight and said compound being stable at 1700 C.

6. An indirectly heated cathode assembly comprising a cathode coated with electron emissive oxides, and a heater for said cathode comprising tungsten wire and a coherent insulating layer interposed between said cathode and said wire consisting predominantly of chemically pure alumina and the remainder a stable and refractory oxygenic compound of barium containing 0 from A% to 1% of the coating by weight of 7 barium.

GEORGE R. SHAW. 

